How can I check which of my collections are hidden in viewport?
Looping collections through bpy.data.collections gives you classes of type Collection:
https://docs.blender.org/api/current/bpy.types.Collection.html#bpy.types.Collection
Looping collections through bpy.context.view_layer.layer_collection.children gives you classes of type LayerCollection:
https://docs.blender.org/api/current/bpy.types.LayerCollection.html#bpy.types.LayerCollection
The meaning of the property hide_viewport seems to be slightly different in these two class types.

In type Collection it means: &quot;Globally disable in viewports&quot;
In type LayerCollection it means: &quot;Temporarily hide in viewport&quot;
Type LayerCollection additionally exposes the property is_visible, which is True, when the collection is both enabled and visible

I guess LayerCollection.hide_viewport is what you were looking for.
Code example to see the differences in action:
print('Looping type &quot;Collection&quot;, property &quot;hide_viewport&quot;:')
for collection in bpy.data.collections:
    print(collection.name, ' - Disabled in Viewport:', collection.hide_viewport)

print('Looping type &quot;LayerCollection&quot;, property &quot;hide_viewport&quot;:')
for layer_collection in bpy.context.view_layer.layer_collection.children:
    print(layer_collection.name, ' - Hidden in Viewport: ', layer_collection.hide_viewport)

print('Looping type &quot;LayerCollection&quot;, property &quot;is_visible&quot;:')
for layer_collection in bpy.context.view_layer.layer_collection.children:
    print(layer_collection.name, ' - Enabled and visible: ', layer_collection.is_visible)